Extramammary Paget disease or EMPD is a rare skin cancer that presents with a red patch in the vulva or anus in women, and in the scrotum, penis and anus in men. Diagnosing EMPD is challenging because the entire area affected by the disease is not visible with the naked eye, or can be misdiagnosed with other skin conditions such as infection or inflammation. This is particularly important when monitoring treatment response, as treatments can irritate the skin and look identical (red patch) to residual EMPD.

Reflectance confocal microscopy or RCM is an imaging system which uses a light source that does not damage the skin and that allows to see cells on the superficial layers on the skin. RCM has been used to diagnose EMPD as one can see the cancer cells in the skin in real time. However, RCM has not been evaluated to identify if EMPD remains active after therapy. Recently we have published in JAMA Dermatology the first study using RCM in patients with previously treated EMPD. We studied 5 patients with previously treated EMPD (4 men and 1 woman). We evaluated 22 sites in the skin suspicious for EMPD using RCM and later obtaining a skin biopsy of these sites. We have found that if we saw EMPD using RCM, in all the cases (9 out 9) the biopsy revealed active EMPD. On the other hand, in the cases where we did not see cancer using RCM, there was a small chance of missing active EMPD (3 out of 13 sites). We believe this may happen because EMPD cells may form small groups of cells that can be difficult to see with RCM, especially in cases which had received therapy, or may be located too deep to be seen with RCM.

Our study has shown that RCM can be very useful to identify EMPD after treatment, and eventually if one sees EMPD on RCM, biopsies may not be necessary and can be spared in such a sensitive location. However, because there is a small chance of missing disease recurrence if RCM does not see cancer, biopsies may be needed in this setting. To sum up, we believe that by assessing previously treated EMPD with RCM, our patients will need less biopsies to know if their disease is still active or not, and whether further treatment is required.